1. Field of the Invention
The present invention relates to electrical circuit protection.
2. Introduction to the Invention
Ground fault interrupters (GFIs) are widely used to provide protection from the harmful effects which can result when there is leakage from an electrical circuit to ground. GFIs compare the current flowing at two different locations in a circuit, and interrupt the circuit if the currents differ by more than a predetermined value, e.g. as the result of a ground fault between the locations. GFIs do not, however, protect against faults which do not result in such a current imbalance, e.g. an overcurrent resulting from a short within the load, an overvoltage arising from lighting, electrostatic discharge, switching of reactive loads, etc., or a current or voltage having a frequency different from, e.g. higher than, the excitation frequency (e.g., 50 Hz or 60 Hz) of the power source of the circuit.
There are many circumstances in which a circuit may be subject to a current or voltage which has a frequency other than the excitation frequency and which is potentially harmful. For example, devices such as motors, phase controllers, non-linear loads and DC power supplies can inject or reflect currents at harmonics (i.e., multiples) of the excitation frequency. Other devices, such as inverters, non-linear loads and switching power supplies can cause currents at high frequencies which are not necessarily harmonics of the excitation frequency. In some circumstances it is desirable to protect a load from potential harmful effects of such high or low frequency currents. In some circumstances it is desirable to protect the power source, particularly, for example, from harmonics generated by reactive loads which result in a reduction of the power factor.
In addition, the presence of even modest currents at high frequencies can be indicative of an unusual condition which should be corrected. For example, some self regulating heating cables containing conductive polymers, if improperly installed or operated, can generate arcs which have a broad frequency spectrum.
Therefore, there is a need to protect electrical circuits from currents or voltages at frequencies different from the excitation frequencies of the circuits.
We have been investigating the use of GFIs in arrangements which provide overcurrent and/or overvoltage protection in electrical circuits in addition to protection from ground faults. A number of such arrangements are disclosed in the earlier applications incorporated by reference herein. We have discovered, in accordance with the present invention, that very useful frequency-selective protection systems can be produced using GFIs. Such frequency-selective systems can be configured to protect from currents or voltages at frequencies above a selected frequency (referred to in this specification as high-pass), and/or from currents or voltages at frequencies below a selected frequency (refereed to in this specification as low-pass) and/or from currents or voltages at frequencies within a selected frequency range (referred to in this specification as band-pass). As used in this specification: (i) xe2x80x9cfrequency-selective protectionxe2x80x9d refers to protection from currents and/or voltages in one or more selected high-pass, low-pass or band-pass ranges; and (ii) xe2x80x9cfrequency-selective currentxe2x80x9d and xe2x80x9cfrequency-selective voltagexe2x80x9d refer respectively to a current or voltage having a frequency in one or more selected high-pass, low-pass or band-pass ranges.
As used herein, the frequency response of a GFI circuit refers to the response of the GFI circuit to a current imbalance as a function of frequency, as compared with the response of the GFI circuit to a current imbalance at the excitation frequency of the power supply. As used herein, the frequency response range of a GFI circuit refers to the frequency range, above and/or below the excitation frequency of the power supply, within which the GFI circuit will trip on a specified current imbalance level. As used herein, a specified current imbalance level refers to a current imbalance level at which it is intended for a particular frequency-selective protection arrangement to cause the GFI circuit in that arrangement to trip.
In one embodiment of the invention, frequency-selective current protection is provided by connecting a control element in series with the line path (or return path) of a GFI, and a bypass element in parallel with the combination of the control element and the line path (or return path) of the GFI. Under normal current conditions, little or no current flows through the bypass element. However, the frequency response characteristics of the control and bypass elements are such that a frequency-selective component (i.e. high-pass, low-pass or band-pass) of the current in the circuit is diverted through the bypass element. Therefore, when the magnitude of such a frequency-selective component reaches a predetermined value, a resulting current imbalance in the GFI causes the GFI to trip. Circuit protection systems in accordance with this first embodiment are particularly useful to protect against currents at frequencies which are within the frequency response range of typical GFI circuits, e.g. as high as 1 kHz, preferably as high as 10 kHz, especially as high as 25 kHz This facilitates the use of GFI circuits, without modification, in systems of the invention.
In a second embodiment of the invention, frequency-selective voltage protection is provided by connecting a bypass element: 1) from the line path input, through a GFI transformer, to the return path output of the GFI; 2) from the line path input to the return path input of a GFI; 3) from the line path output through a GFI transformer, to the return path input of the GFI; or 4) from the line path output to the return path output of a GFI. Under normal conditions, little or no current flows through the bypass element However, when a frequency-selective voltage above a predetermined voltage value is present across the bypass element, an increased current passes through the bypass element creating a current imbalance in the GFI and causing the GFI to trip. Circuit protection systems in accordance with this second embodiment are particularly useful to protect against voltages at frequencies which are within the frequency response range of typical GFI circuits, again facilitating the use of GFI circuits, without modification, in systems of the invention.
In a third embodiment of the invention, frequency-selective current protection is provided by connecting (i) a bypass element: 1) from the line path input, through a GFI transformer, to the return path output of the GFI; 2) from the line path input to the return path input of a GFI; 3) from the line path output through a GFI transformer, to the return path input of the GFI; or 4) from the line path output to the return path output of a GFI; and (ii) a control element in the line (or return line) of the circuit. Under normal conditions, little or no current flows through the bypass element. However, when a frequency-selective current above a predetermined current value is present, the control element causes the bypass element to pass a portion of the circuit current, thereby creating a current imbalance in the GFI and causing the GFI to trip. Circuit protection systems in accordance with this third embodiment can protect against currents at frequencies within the frequency response range of typical GFI circuits. In addition, however, they are also useful to protect. against currents at frequencies which are outside the frequency response range of typical GFI circuits, again facilitating the use of GFI circuits, without modification, in systems of the invention. This feature is facilitated by the use of a control element which is capable of detecting a current at a frequency outside the frequency response range of the GFI circuit When the control element detects a current within a frequency-selective range, and above a predetermined current value, the control element causes the bypass element to pass a portion of the circuit current, which includes current at the excitation frequency of the power source, i.e. within the frequency response range of the GFI circuit As a result, the GFI trips on the current imbalance. In this manner, the magnitude of the imbalance current which passes through the bypass element is independent of the magnitude of the frequency-selective current which triggers the control element.
In a fourth embodiment of the invention, frequency-selective voltage protection is provided by connecting (i) a bypass element: 1) from the line path input, through a GFI transformer, to the return path output of the GFI; 2) from the line path input to the return path input of a GFI; 3) from the line path output through a GFI transformer, to the return path input of the GFI; or 4) from the line path output to the return path output of a GFI; and (ii) a control element from the line to the return line, in parallel with the load. Under normal conditions, little or no current flows through the bypass element. However, when a frequency-selective voltage is present across the control element, and exceeds a predetermined voltage value, the control element causes the bypass element to pass a portion of the circuit current, thereby creating a current imbalance in the GFI and causing the GFI to trip. Circuit protection systems in accordance with this fourth embodiment can protect against voltages at frequencies within and/or outside the frequency response range of typical GFI circuits, and again facilitate the use of GFI circuits, without modification, in systems of the invention.
Thus, in accordance with the invention, commonly available GFI devices may be used to provide integrated frequency-selective protection in electrical circuits, devices and systems, in addition to protection from ground faults.
In the several embodiments of the invention, frequency-selective current and/or voltage protection is provided using commonly available GFI circuits without modification. However, this is not to preclude the modification of such GFI circuits, for example, to expand the frequency-response range of such GFI circuits, or to amplify the response of the GFI circuits within a frequency-selective range of interest. Such modifications may include, for example, the use of different GFI transformers or other components in the GFI circuit.
In general, the control and bypass elements comprise electrical components arranged to form filter circuits. Such filter circuits may range from fairly simple to complex tuned circuits, and can be used individually or in combination to produce low-pass, high-pass, band-pass and combination filters with steep or gradual roll-off characteristics. While many such filter circuits are known, we have discovered such circuits can be used in combination with GFI circuits in accordance with the present invention to provide frequency-selective circuit protection. Moreover, while the filter circuits shown in the examples of the several embodiments of the invention described herein are generally simple combinations, such is intended to be merely by way of example and is not intended to limit or exclude the use of more complex filter circuits.
The term xe2x80x9cground faultxe2x80x9d is used in this specification to denote any fault, external to a comparison element such as a GFI, which causes a current to flow in one sense line of the comparison element, which is not matched by an equal and opposite current in the other sense line of the comparison element. The term xe2x80x9cexternal to a comparison elementxe2x80x9d is used in this specification to refer to any point in a circuit between the line sense output of the comparison element and the return sense input of the comparison element.
The protection systems of the invention may be configured to provide both frequency-selective current and frequency-selective voltage protection in addition to the normal ground fault interrupt protection provided by the GFI or other comparison element. They may also be configured to provide only frequency-selective current or frequency-selective voltage protection in addition to the ground fault interrupt protection.
The protection systems of the invention may also be configured with remote control and switching capability. For example, a protection system may be configured to trip on a tone at a particular frequency, which tone may be remotely injected onto the line to cause the protection system to trip. For example, a utility company could remotely disconnect a consumer either to test the connection or for non-payment. In addition, a circuit protection system (overcurrent, overvoltage, frequency-selective) at one location could remotely trip a circuit protection system at another location. For example, if each plug in a circuit were set to trip at a different current level, each trip detector could be configured to send a tone back to the circuit breaker to cause it to interrupt. Thus, each circuit could have its trip current individually set, but only one central interrupting breaker would be required.